Fix Azure name cleanup

[nitely]

fixes #609

Changes:

• remove stripping special characters from file name
• escape URL reserved and unsafe characters on url method

Notes:

• This is consistent with the other storages
• Django does the same name cleaning as the previous code, anyway
• Django name cleaning can be bypassed by overriding get_valid_name or calling default_storage.save directly. This was not possible with the old code
• Since this is a superset of what was previously allowed, it's a minor breaking change
• Files uploaded with the previous code will still work

[jschneier]

@nitely is this still necessary with your other branch now merged?

[nitely]

Yes, this removes some silly restrictions in file names. I made anew PR with all conflicts resolved #752